Missing rust_begin_unwind symbol for msp430 (debug-only?) builds

[cr1901]

Code

Instructions

1. Make sure the msp430-elf-gcc toolchain is installed. Optionally install just for convenience.
2. git clone https://github.com/cr1901/msp430-size. Use commit 036da7a specifically.
3. Install any rust nightly after 2022-09-08 (I am using 2022-09-21/9062b780b32d2eab060b4432863e085d9504ca5c for the error below).
4. Run the following command:

   cargo +override-name rustc --manifest-path=./test-cases/Cargo.toml --target=msp430-none-elf -Zbuild-std=core --features=unsafe -- --emit=obj=target/msp430-none-elf/debug/test-cases.o,llvm-ir=target/msp430-none-elf/debug/test-cases.ll,asm=target/msp430-none-elf/debug/test-cases.s

With a working compiler (I used 2022-08-10/34a6cae28e7013ff0e640026a8e46f315426829d for this example), I expect compilation to succeed:

   Compiling test-cases v0.1.0 (/home/william/Projects/embedded/msp430/msp430-size/test-cases)
warning: unused import: `interrupt as mspint`
  --> test-cases/src/main.rs:10:15
   |
10 | use msp430::{{interrupt as mspint}, interrupt::CriticalSection};
   |               ^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

warning: `test-cases` (bin "test-cases") generated 1 warning
    Finished dev [unoptimized + debuginfo] target(s) in 0.20s

Instead, I get a link error about a missing rust_begin_unwind symbol, and cannot link any code that doesn't optimize it out (so everything with opt-level=0 and sometimes higher opt-levels?):

Compiling test-cases v0.1.0 (/home/william/Projects/embedded/msp430/msp430-size/test-cases)
warning: unused import: `interrupt as mspint`
  --> test-cases/src/main.rs:10:15
   |
10 | use msp430::{{interrupt as mspint}, interrupt::CriticalSection};
   |               ^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(unused_imports)]` on by default

error: linking with `msp430-elf-gcc` failed: exit status: 1
  |
  = note: "msp430-elf-gcc" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/test_cases-77e7fc9bffc4f0a7.s693teeolgqnrfj.rcgu.o" "-L" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps" "-L" "/home/william/Projects/embedded/msp430/msp430-size/target/debug/deps" "-L" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/build/test-cases-3f682f73842f7eb2/out" "-L" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/build/msp430-rt-b95e8a1acafa0902/out" "-L" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/build/msp430g2553-3b2ab7604a829bcc/out" "-L" "/home/william/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/msp430-none-elf/lib" "-Wl,-Bstatic" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libmsp430g2553-ffaf04726949f42c.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libvcell-d1c6da43e237512b.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libmsp430_rt-10b834e6ef56ed17.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libcfg_if-092783056b17116d.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libpanic_msp430-f198507bed824f51.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libmsp430-e970b409efe93be8.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libcritical_section-7116d7524c9bc4b8.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/librustc_std_workspace_core-a9d0ab60345003e9.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libcore-7b26ee1f55864a47.rlib" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libcompiler_builtins-3594f08dc3904fc7.rlib" "-Wl,-Bdynamic" "-L" "/home/william/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/msp430-none-elf/lib" "-o" "/home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/test_cases-77e7fc9bffc4f0a7" "-nodefaultlibs" "-Tlink.x" "-nostartfiles" "-mcpu=msp430" "-lmul_none" "-lgcc"
  = note: /home/william/.local/bin/../lib/gcc/msp430-elf/9.2.0/../../../../msp430-elf/bin/ld: /home/william/Projects/embedded/msp430/msp430-size/target/msp430-none-elf/debug/deps/libcore-7b26ee1f55864a47.rlib(core-7b26ee1f55864a47.core.a947c771-cgu.0.rcgu.o): in function `core::panicking::panic_fmt':
          /home/william/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/panicking.rs:142: undefined reference to `rust_begin_unwind'
          collect2: error: ld returned 1 exit status
          
  = help: some `extern` functions couldn't be found; some native libraries may need to be installed or have their path specified
  = note: use the `-l` flag to specify native libraries to link
  = note: use the `cargo:rustc-link-lib` directive to specify the native libraries to link with Cargo (see https://doc.rust-lang.org/cargo/reference/build-scripts.html#cargorustc-link-libkindname)

warning: `test-cases` (bin "test-cases") generated 1 warning
error: could not compile `test-cases` due to previous error; 1 warning emitted
error: Recipe `build-example` failed with exit code 101

The msp430-none-elf target doesn't support unwinding (and I don't imagine it ever will), only aborting panics. So I'm not sure why rust_begin_unwind is being linked in in the first place (or how compilation should be fixed).

cargo-bisect-rustc output (Version it worked on/Version with regression)

searched nightlies: from nightly-2022-08-07 to nightly-2022-09-21
regressed nightly: nightly-2022-09-08
searched commits: from 78a891d to c2804e6
regressed commit: 699bfa8

bisected with cargo-bisect-rustc v0.6.0

Host triple: x86_64-unknown-linux-gnu
Reproduce with:

cargo bisect-rustc --preserve-target --preserve --start=2022-08-07 --end=2022-09-21 --with-src -- build -Zbuild-std=core --target=msp430-none-elf --features=unsafe 

[apiraino]

WG-prioritization assigning priority (Zulip discussion).

thanks @cr1901 for the bisection. It leads to #100404 cc: @belovdv

@rustbot label -I-prioritize +P-medium

[petrochenkov]

This issue should be fixed by #103092.
Could you test it once that PR lands into nightly?

This is a different issue.

[petrochenkov]

This should be fixed by #103240.

[cr1901]

I rebuilt Rust w/ #103240 and then tested against the code I linked in the initial post, and linking in debug mode succeeded; #103240 will indeed fix the issue.